Warn as well if no directory name for cachedir provided

cachedir is used to determine where cache files are stored.
the empty directory will ends up to put them under the top of
directory of XDG_CACHE_HOME. that messes it up and must be avoided.

As a note, if you want to read/write something from the top of
XDG_DATA_HOME (dir) and/or XDG_CACHE_HOME (cachedir),
put "." instead of the empty.

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